Analysis of immune responses induced by avian pathogenic Escherichia coli infection in turkeys and their association with resistance to homologous re-challenge.

Avian pathogenic Escherichia coli (APEC) cause severe respiratory and systemic disease in poultry yet the nature and consequences of host immune responses to infection are poorly understood. Here, we describe a turkey sub-acute respiratory challenge model and cytokine, cell-mediated and humoral responses associated with protection against homologous re-challenge. Intra-airsac inoculation of turkeys with 105 colony-forming units of APEC O78:H9 strain χ7122nalR induced transient and mild clinical signs of colibacillosis followed by clearance of the bacteria from the lungs and visceral organs. Upon re-challenge with 107 χ7122nalR, primed birds were solidly protected against clinical signs and exhibited negligible bacterial loads in visceral organs, whereas age-matched control birds exhibited high lesion scores and bacterial loads in the organs. Levels of mRNA for signature cytokines suggested induction of a Th1 response in the lung, whereas a distinct anti-inflammatory cytokine profile was detected in the liver. Proliferative responses of splenocytes to either Concanavalin A or soluble χ7122nalR antigens were negligible prior to clearance of bacteria, but APEC-specific responses were significantly elevated at later time intervals and at re-challenge relative to control birds. Primary infection also induced significantly elevated χ7122nalR-specific serum IgY and bile IgA responses which were bactericidal against χ7122nalR and an isogenic Δrfb mutant. Bactericidal activity was observed in the presence of immune, but not heat-inactivated immune serum, indicating that the antibodies can fix complement and are not directed solely at the lipopolysaccharide O-antigen. Such data inform the rational design of strategies to control a recalcitrant endemic disease of poultry.